1. Field of the Invention
The present invention relates to a casting method of a silicon ingot and a cutting method of the same, more specifically to the casting method and cutting method of the silicon ingot that enhance the manufacturing efficiency, production yield of the silicon ingot and conversion efficiency of a solar battery using a silicon block cut out from the silicon ingot as a substrate material.
2. Description of the Related Art
The majority of solar batteries manufactured these days use silicon crystals as substrate materials for them. The silicon crystal is classified into a monocrystal and a polycrystal. Generally, the solar battery having a high efficiency of energy conversion from incident light energy into electric energy can be obtained by using the monocrystal as the substrate.
Since a high-quality dislocation-free crystal is required for manufacturing a monocrystal silicon, the Czochralski method that pulls up and grows the monocrystal from molten silicon is applied for the production. However, the monocrystal silicon grown by the Czochralski method increases the manufacturing cost compared to the polycrystal silicon described later, which is disadvantageous. Accordingly, using the monocrystal silicon as the substrate of the solar battery will increase the manufacturing cost of the solar battery, which is of an issue.
On the other hand, the polycrystal silicon is generally manufactured by a casting method that solidifies the molten silicon with a mold (hereinafter referred to as ‘casting method’), or a continuous casting method by electromagnetic induction (hereinafter referred to as ‘electromagnetic casting method’). This casting method or the electromagnetic casting method is capable of manufacturing a substrate material at a lower cost than the monocrystal silicon substrate manufactured by the Czochralski method.
In casting the polycrystal silicon by the casting method, a high-purity silicon as a raw material is heated and melted inside a crucible, and a trace amount of boron etc. as a doping material is uniformly added, and thereafter the molten metal is solidified inside the crucible or poured into the mold to be solidified therein. Generally, a silicon block is a square, from which the substrate used for the solar battery is sliced. Accordingly, in case of solidifying the molten metal as it is after melting, a square quartz crucible is used; and in case of pouring the molten metal into the mold, a square graphite mold is used.
Applying a one-directional solidification method to this casting method makes it possible to obtain a polycrystal silicon of a large crystal grain. However, since the casting method is an ingot making method that solidifies the molten silicon with the mold, there arise various problems. Contacting the molten silicon with a vessel wall, for example, might cause an impurity contamination. Further, a mold lubricant used for preventing seizure of an ingot with the mold might be mixed into the molten silicon.
As mentioned above, the casting method requires using high-purity materials for the quartz crucible and the graphite mold etc. as well as replacing these periodically, which leads to an increase of the manufacturing cost. Further, the casting method is the ingot making method and has difficulties in casting continuously, which will incur a decrease in the manufacturing efficiency.
As a method for solving these problems, an electromagnetic casting method is developed, which is capable of casting the silicon crystal in such a manner that the molten silicon does not substantially come into contact with the crucible and the mold.
The electromagnetic casting method uses an apparatus where parts each in a strip shape, made of materials with high electrical conductivity and thermal conductivity, electrically insulated to each other in a circumferential direction, and water-cooled inside thereof, are disposed inside a high-frequency induction coil. In regard to the cross-sectional shape of the coil and a body enclosed with the strip-formed elements constituting the crucible, both a circular cylindrical shape and a rectangular cylindrical shape are applicable.
Since each of the strip-formed elements constituting the cooling copper crucible is electrically separated from each other, when a raw material silicon is charged into the cooling copper crucible as a melting vessel and an alternate current is applied to the high-frequency induction coil, a current loop is generated inside each element. Since the current on the side surface of an inner wall of the cooling crucible forms a magnetic field inside the crucible, the silicon inside the crucible can be heated and melted. The silicon inside the crucible receives force (pinch force) inwardly in a direction normal to the surface of the molten silicon by the interaction between a magnetic field formed by the current on the inner wall of the cool crucible and a skin current of the molten silicon; thereby, the silicon is melted in a non-contact state relative to the crucible.
The electromagnetic casting method uses the water-cooling copper crucible having been used for melting also for solidification. Specifically, moving downward a supporting base that holds the molten silicon and the ingot on its lower part while melting the silicon inside the crucible will decrease an induced magnetic field, as the supporting base moves away from the lower end of the high-frequency induction coil; therefore, a calorific value and the pinch force attenuate, and further the cooling by the water-cooling copper crucible will progress the solidification from an outer circumferential portion of the molten silicon. By continuously inputting the raw material from the upper part of the crucible to continue the melting and solidification according to the downward movement of the supporting base, the polycrystal silicon can be continuously cast while solidifying from the lower part of the water-cooling copper crucible, without contacting the molten silicon with the wall of the crucible.
As mentioned above, the electromagnetic casting method has advantages that: the molten silicon hardly comes into contact with the crucible; the use of the high-purity material for the crucible is not necessary; and the cooling area is wide to thereby make it possible to increase the casting speed. From the viewpoint of these advantages, various examinations have been made for enhancing the quality of the solar battery using the polycrystal silicon as the substrate material as well as for lowering the manufacturing cost.
The Japanese Patent Application Publication No. 2-51493 discloses an apparatus by the electromagnetic casting method using the above cooling crucible, comprising the construction of filling a closed vessel with an inert atmosphere pressured slightly higher than the atmospheric pressure, continuously pulling out a continuously cast ingot by providing a substantially non-contact seal on the lower part of the closed vessel, and mechanically cutting the ingot at a position exited from the closed vessel. Thereby, although the conventional method has been restricted by the capacity of an electromagnetic casting furnace and the casting could be performed only intermittently until now, it becomes possible to continuously produce the silicon ingot.
Besides, various examinations have been made not only for the continuous casting process but also for the process of manufacturing silicon blocks from a cast ingot, in order to enhance the quality of the solar battery and reducing the manufacturing cost.
FIG. 1 typically shows a crystallized pattern of a longitudinal section of an ingot cast by using an electromagnetic casting method. As shown in FIG. 1, in the crystallized pattern of the longitudinal section of the ingot, a chill layer 1 of a fine crystal grain size grows from a side surface of the ingot in a direction perpendicular to the side surface, and columnar crystals 2a, 2b grow to thicken toward the upper heat source, exhibiting much conspicuous tendency at the inner portion thereof.
The inside of the ingot containing the columnar crystals 2a, 2b with a larger crystal grain size grown has an excellent semiconductor characteristic; however, since the chill layer 1 has a smaller grain size and has abundant crystal defects, the semiconductor characteristic thereof cannot be said excellent. Accordingly, it is necessary to excise a certain portion with a certain thickness (hereinafter also referred to as ‘edge’) from the side surface of the ingot for removing the chill layer 1. Generally, the above excision of the edge is executed in cutting out the silicon blocks from the ingot.
FIGS. 2A and 2B typically explain a conventional method of cutting out silicon blocks from a silicon ingot, in which FIG. 2A is a conceptual view showing the direction of cutting out the silicon blocks, and FIG. 2B is a sectional view of the silicon ingot. As shown in FIG. 2A, since the substrate of a solar battery assumes a square, the conventional manufacturing method of the silicon blocks adopts a method for cutting out four pieces of silicon blocks 5 with a square section from an ingot 4 cast in a square sectional shape.
As shown in FIG. 2B, after an edge 6 forming the chill layer on the side surface is excised, the ingot 4 is cut and divided into four pieces of the silicon blocks 5 along a cutting margin 7. Thus, excising the edge 6 will produce the silicon blocks having a large crystal grain size and excellent semiconductor characteristics with few crystal defects.
As mentioned above, in the cutting method of the silicon ingot using the conventional electromagnetic casting method, a reasonable method for preventing the production yield from lowering is applied in consideration of the aspect that the substrate of the solar battery is used in a square shape.